Titanium pretreatment for protective coating of refractory alloys



United States Patent 3,268,3fl6 TITANIUM PRETREATh EENT FOR PROTECTHVE COATING OF REFRACTORY ALLOYS Richard Alexander .lefferys, Euclid, Ohio, assignor to TRW Hue, a corporation of Ohio No Drawing. Filed Mar. 28, 1962, Ser. No. 353,062 4 Claims. (Ci. 2194) The present invention generally relates to metal alloys and more particularly to improved refractory oxidation resistant metal alloys for high temperature applications and methods of producing the improved refractory oxidation resistant metal alloys.

Refractory metals and alloys are not generally useful as structural materials for hi h temperature applications because of their lack of resistance to oxidation.

The desirability of refractory, oxidation resistant alloys in missiles, rockets and related applications is readily appreciated. Great strides in this direction have been made by protecting the refractory metal or alloy base with an oxidation resistant alloy coating layer of chrmium and titanium. The chromium-titanium may thereafter alloy with another metal or alloy to enhance the oxidation resistance of the metal or alloy base in addition to enhancing other metallurgical properties, such as resistance to thermo shock, impact, abrasion, ablation, and creep. These other alloying metals include, for example, silicon, platinum, aluminum, iron, nickel, cobalt, boron, and alloys formed between refractory materials, included in the class of transition metals of the fourth to sixth groups of the periodic system of elements (titanium, zirconium, hafnium, niobium, tantalum, chromium, molybdenum, tun sten and alloys thereof).

It is generally known that at temperatures about 1800 F. most of the refractory metals, when exposed to atmospheric oxidation, form destructive oxide derivatives, some of which are volatile, some of which are susceptible to creep rupture, some of which become e mbrittled, and some of which suffer strength losses. Moreover, the short transition period required for those undesirable characteristics to appear and to maximize still further render unsuitable the refractory metals for many purposes, since they fail to meet the additional requirements of extended-use life in an oxidant atmosphere at high temperatures, such as encountered in missiles, rockets, and related applications.

The origin of failure of the refractory oxidation resistant alloys known prior to the present invention, may vary from cracks or pin holes produced by thermo or mechanical stresses to the eventual penetration of the surface coating layer by diffusion of oxygen. However, the mode of failure appears to be when sufficient oxygen reaches the base metal 'beneath the coating to form to refractory metal oxide. The volume expansion of the oxide as it forms produces high localized stresses in the regions adjacent to the site. The forces exerted outward produce a shear failure in the coating layer. Once a sufficient passage is open for oxygen into the base metal scale growth proceeds at a rate dependant upon the scaling characteristics of the refractory base metal and rupture of the coating continues in areas adjacent to the original failure point producing a larger and larger defect. At the same time oxygen is penetrating into the base metal forming a sub-scale region.

It is therefore an object of the present invention to provide an improved refractory, oxidation resist-ant metal or alloy and a method of producing the refractory oxidation resistant metal or alloy.

It is further an object of the present invention to provide a refractory oxidation resistant metal or alloy but has been pretreated with titanium.

It is still another object of the present invention to pretreat a refractory metal or alloy base with titanium and thereafter deposit an alloy layer to provide an intermediate alloy layer that prevents diffusion outward of the base metal therethrough and prevents diffusion of oxygen inwardly therethrough.

The present invention involves the forming of an alloy intermediate layer that prevents the diffusion of base metal outwards and the diffusion of oxygen inwards. This is accomplished by pre-alloying the refractory metal or alloy base with titanium. The refractory metal i.e. columbium (niobium) tantalum, molybdenum, and tungsten o-r alloy thereof forms a substitutional surface alloy with titanium. The titanium pre-alloyed refractory metal or alloy base is then coated wit-h a chromium-titanium coating.

The titanium pre-alloying treatment lowers the melting point of the base metal thereby lowering the activation energy required for diffusion. The activation energy for self-diffusion in metals is a direct function of the metal melting point. Alloy additions which lower the melting point of the base metal lower the energy requirements for self-diffusion and also chemical interdiffusion. The titanium which lowers the melting point of the base metal in the region of the surface, effectively lowers the activation energy for chromium diffusion and therefore increases the rate of said chromium diffusion.

Alloying in this way, therefore, produces a refractory base metal or alloy, a diffusion alloy layer of the base metal or alloy with titanium, having thereover a diffusion alloy layer of the base metal or alloy and chromium, having thereover a layer of chromium and titanium. The layer of refractory metal or alloy base and chromium phase is quite stable and prevents the outward diffusion of base metal therethrough and the inward diffusion therethrough of oxygen. The resulting alloy is diagrammatically illustrated as follows:

Surface coatingSi, Pt, Al, Fe, Ni, Co, etc. (see above) Over layer of chromium and titanium Diffusion layer of base and chromium Diffusion layer of base and titanium Baserefractory met-a1 or alloy The base metal or alloy to which the coating is to be applied should have the lowest scaling and intern-a1 penetration possible. With a low scaling rate, the formation of undesirable oxides at the coating-base met-a1 interface is retarded, and, consequently, the time to complete failure of the refractory, oxidation resistant alloy would be correspondingly increased.

Multiphase formations as described above must not be of an undesirable low melting point or high brittleness, otherwise desirable metallurgical properties other than oxidation resistance would be lost. Conversely, phases or compounds formed by an aging process would be desirable, if they are oxidation resistant. Of course, in such a situation, an adequate diffusion layer must be first established to obtain such desirable results.

We have described the present invention as utilizing the coating alloy of chromium and titanium. The coating metals or alloys employed should not only be capable of forming a strong bond with the base metal or alloy, but a bond interface must be formed without spalling, cracking or blistering, which reduces the other desirable mechanical properties of the result-ant alloy, even though the alloy itself may be highly oxidation resistant. The surface layer of the coating and the diffusion layer between the coating and the base metal or alloy or coating adjacent thereto must be oxidation resistant and must present a substantially impervious barrier to oxidation penetration in order to achieve optimum performance results.

The conversion rate of the coating of the resulting alloy to a non-protective oxide derivative must be relatively low as must be the oxidation diffusion rate through the coating to the base metal at the temperatures encountered.

The formation of the refractory, oxidation resistant alloy should preferably occur in a vacuum chamber heated to a temperature suflicient to vaporize the pure coating metal or alloy powders or mixtures thereof in a container to produce a solid-vapor interface diffusion alloying reaction with the base metal or coating thereon to form the surface alloy between the coating and base metal and thus to permit diffusion of the outer surface layer of pure coating metal or alloy.

Since in the practice of my invention a solid-vapor interface reaction occurs between the solid base metal or alloy and the vapor state coating metal or alloy, the diffusion of the vapor into the base metal is in direct proportion to the surface area of the coating metal particles or powders employed or in inverse proportion to the granular size of the particles. Thus the composition of the coating metal vapor atmosphere of the container is a function of the relative proportion of coating metal granules or relative proportion of a mixture of coating metal granules, coating alloy granules or coating alloy mixture granules, the particle size of each type of metal or alloy granule, and the temperature employed which determines the equilibrium vapor pressure in the container. The larger the particle size of the coating metal or alloy, the smaller the surface area thereof, and correspondingly the lower the actual vapor pressure of that metal in the container, due to the lower rate of approach to the equilibrium vapor pressure. When employing a mixture of coating metals, the coating metal powder is preferably of a mesh particle size between l and 200, depending, of course, upon the metals or alloys employed.

The base metal or alloy to be coated is packed in the desired coating metal or alloy or mixtures thereof, which are to produce the metallic vapor atmosphere. The container is fitted with a loosely fitted cap and placed in a Vacuum chamber. Next the vacuum chamber is evacuated preferably to 0.1 micron or less. Again, depending upon the properties of the base metal or alloy relative to the properties of the coating employed, the vapor chamber is heated to a temperature sutficient to vaporize the coating.

The coating atmosphere within the container is the vaporous coating metal or alloy and is uncontaminated by other gases or by water vapor, when equilibrium vapor pressure conditions are established in the container. The base metal therefore undergoes a solid-vapor interface reaction with the vaporous coating which diffuses into the base metal.

It will be appreciated that the composition and the depth of the resulting alloy base-coating diffusion layer is a function of the composition of the coating vapor atmosphere, the relative diffusion rates of the coating metal or alloy or mixtures thereof in the base metal, if more than one is employed, and the time of diffusion at the established temperature. After the desired depth of diffusion has been obtained, the temperature of the chamber is reduced and upon cooling, the vapors of the coating metal form a firmly bonded surface layer of the coating metal or alloy on the diffusion layer.

The composition and the depth of penetration of the final alloy diffusion layer is as aforesaid a function of the composition of the coating vapor atmosphere, the relative diffusion of the coating metals employed into the base metal, and the time allowed for diffusion to occur, at the temperature selected. The composition and thickness of the coating layer surface on the diffusion layer produced on cooling is a function of the composition of the metal vapor atmosphere and the rate of cooling permitted.

For particular resultant, refractory, oxidation resistant alloys, excessive high coating temperatures which may tend to embrittle the base metal alloy or raise its transition temperature must be avoided. The alloy should be formed in the absence of a contaminating atmosphere containing oxygen, nitrogen, hydrogen or Water vapor and in addition employment of inert gas atmospheres. The use of such atmospheric conditions during vaporization suppresses the vaporization of the coating metals.

Further, another metal or alloy may be vapor deposited and alloyed to the chromium-titanium to enhance the oxidation resistance of the metal or alloy base in addition to enhancing other metallurgical properties, such as resistance to thermo shock, impact, abrasion, ablation, and creep. These other alloying metals include, for example, silicon, platinum, aluminum, iron, nickel, cobalt, boron, and alloys formed between said refractory materials included in the class of transition metals of the fourth to sixth groups of the periodic system of elements (titanium, zirconium, hafnium, niobium, tantalum, chromium, molybdenum, tungsten and alloys thereof.

Satisfactory refractory, oxidation resistant alloys have been obtained in employing as a base metal or alloys, binary alloys, ternary alloys and rnulti-component alloys including as constituents thereof from 1 to 10% titanium, 1 to 15% molybdenum, 1 to 5% zirconium, and l to 20% tungsten based on columbium (niobium) content. Table I shows some of the satisfactory refractory metals or alloys satisfactorily used in the present invention.

TABLE I 1 Unalloyed Ch 10 Cb-SW 2 Cb-lZr 11 Cb-5Zr15W 3 Cb-SZr 12 Cb-5Zr10W0.05B 4 C-b-lTi 13 Cb-IOTi-lMo 5 Cb-3Ti 14 Nb-lOTi-AOMO (6) Cb10Ti 15 Cb-l0Ti15Mo 7 Cb-2OTi (16) cum-101x40 s (lb-1W 17 Cb-10Ti5Mo 9 Ob-3W 1s Nb-lOTi-6Mo-2OW (19 Cb-33Ta1Zr Example 1 A specimen of unalloyed columbium was cleaned by degreasing and the clean specimen packed in a columbiurn container (the container is constructed of a material compatible with such metals or alloys used for the base and coatings, and the reaction conditions required for a particular application) with unalloyed titanium granules. The cap was placed on the container, and the container placed in a vacuum chamber. The chamber was evacuated under a vacuum maintained at 0.1 micron and the vacuum chamber is thereafter heated to 2200 F. During evacuation, air is drawn from the container and the atmosphere within the can is unalloyed titanium vapor when equilibrium vapor pressure conditions are established. It will be appreciated that the titanium vapor atmosphere in the container is uncontaminated by other gases such as oxygen, nitrogen, hydrogen, or water vapor. The surface of the columbium specimen undergoes a solid-vapor interface reaction with the titanium vapors such that the titanium vapor alloys directly with and diffuses into the columbium base metal and forms an alloy diffusion zone of titanium and columbium which is evident upon microscopic examination. Since the depth of the diffusion zone is a function of reaction time at reaction temperature, the reaction is permitted to continue for a period of 16 hours at a temperature of 2200 F., to form the diffusion zone.

The temperature was thereafter reduced and upon cooling a layer of titanium was found to have deposited in a firm bond over the diffusion layer when the ambient temperature was reached.

After the cooling, the titanium prealloyed columbium is packed in a mixture of unalloyed chromium metal and unalloyed titanium metal. The coating powder pack may consist from to 40% titanium and 85 to 60% chromium by volume, preferably from to titanium and 80 to 70% chromium (10 +12 mesh particle size and +50 mesh respectively). The cap was placed on the container, and the container placed in a vacuum chamber. The chamber was evacuated under a vacuum maintained at 0.1 micron and the vacuum chamber thereafter heated to 2350 F. During evacuation, air is drawn from the container and the atmosphere within the container is unalloyed chromium and unalloyed titanium vapor when equilibrium vapor pressure conditions are established. It will be appreciated that the chromium and titanium vapor atmosphere in the container is uncontaminated by other gases such as oxygen, nitrogen or hydrogen or water vapor. The titanium surface of the columbium specimen undergoes a solidvapor interface reaction with the chromium and titanium vapors such that the chromium-titanium vapors alloy directly with and diffuse into the pre-alloyed columbium metal and forms a diffusion zone. The reaction was permitted to continue for 16 to 24 hours.

The high titanium concentration produced in the prealloying treatment permitted the diffusion of more chromium into the diffusion zone during the chromiumtitanium treatment; this produced a higher concentration of chromium than was previously possible. The coating time and temperature permitted the columbium base to diffuse outwardly through the columbium-titanium diffusion zone and the chromium to diffuse inwardly from the chromium-titanium diffusion zone and form an intermediate diffusion zone of columbium and chromium (CbCr which is apparent upon microscopic examination.

The temperature was thereafter reduced and upon cooling a layer of chromium-titanium alloy was found to have been deposited in a firm bond over the diffusion layer of columbium and chromium.

The resultant alloy had a layer of base columbium, an alloy diffusion zone of titanium and columbium, followed by a layer of a second phase of columbium and chromium, and a top layer of chromium-titanium alloy.

Example 2 An unalloyed columbium base was prealloyed with titanium in the same manner as set forth in Example 1. It was then alloyed in accordance with the procedures of Example 1 with a chromium-titanium coating alloy at a temperature of 2350 F. for 16 hours. After the cooling, the resulting alloy was packed in silicon metal powder (100 to +150 mesh particles size) and the temperature of the vacuum chamber raised to a level sufficient to vaporize the silicon, about 2150 F. The silicon reaction was permitted to continue for a period of 16 hours at 2150 F. Examination of the resultant alloy, by both an optical and electron microscope and an X-ray diffraction identification, well known in the prior art, indicated that the application of silicon to the chromium-titanium prealloyed titanium base further increase the coating life at temperatures above 2250 F. in the form of an additional barrier to the oxygen comprising a diffusion zone of silicon-chromium-titanium and a top layer of silicon.

Upon examination, the resultant columbium-titaniumchromium-silicon alloy comprises a layer of base columbium, an alloy diffusion zone of titanium and columbium, a layer of a second phase of columbium chromium, a

6 layer of titanium-chromium alloy, a diffusion alloy zone of titanium-chromium-silicon alloy, and an outer layer of silicon metal. The test conducted on a resultant refractory oxidation resistant alloy indicated that increased coating life was obtained as compared to the resultant alloy of Example 1.

Example 3 A Cb-10Ti-6Mo-20W alloy was prealloyed with titanium and coated with a titanium-chromium alloy by the process of Example 1.

The alloy formed upon examination was the same as that of Example 1 with the unalloyed columbium replaced by the above columbium base alloy.

Example 4 A Cb-l0Ti-6Mo-20VV base alloy was preailoyed with titanium, then coated by my vacuum vapor deposition method with chromium-titanium alloy and then alloyed with silicon by the process of Example 2.

The alloy formed Was the same as that of Example 2 except the unalloyed columbium was replaced by the above columbium base alloy.

Example 5 An unalloyed columbium base coated the same as Example 1 except the prealloying With titanium is omitted.

The alloys formed by Example 5 show a columbium base having an alloy diffusion zone of titanium-columbium, a diffusion zone of titanium-chromium-columbium, and an outer layer of titanium-chromium alloy.

Example 6 An unalloyed columbium base alloyed the same as Example 2 except the titanium prealloying step is omitted.

The resultant alloy showed a columbium base having an alloy diffusion zone of titaniumcolumbium, a diffusion zone of titanium-chromium-columbium, a layer of titanium-chromium, a diffusion zone of titanium-chromiumsilicon, and an outer layer of silicon.

Example 7 A Cb-lOTi-6Mo-2OW alloy base was coated with chromium-titanium the same as in Example 3 except the titanium prealloying step was omitted.

The alloy formed was the same as the alloy of Example 5 with the unalloyed columbium replaced by the above columbium alloy.

Example 8 A Cb-lOTi-6Mo-20W alloy base was coated with chromium-titanium and then silicon in the same way as in Example 4 except the titanium prealloying step was omitted.

The alloy formed was the same as the alloy of Example 6 with the unalloyed columbium replaced by the above columbium alloy.

The alloys produced by Examples 1-8 after having been cooled to ambient temperatures, were subjected to thermal shock tests. The tests involved exposing the refractory, oxidation resistant alloy alternately to a highly oxidized oxy-acetylene flame and to an air blast for cooling thereof.

The alloys were also deformed in conventional bend oxidation and creep rupture tests under elongations of from between 5% to about 10% at temperatures of about 2500 F.

To test oxidation resistance of the alloy coating, refractory, oxidation resistant alloys prepared in accordance with the above procedures were subjected to a number of cooling cycles involving temperatures within the range of 2500 F. Results were compared and it showed that the life of the prealloyed titanium-columbium article produced in Examples 1 and 2 was increased 4 or 5 times over the columbium base not prealloyed with titanium, as produced in Examples 5 and 6. By comparing the prealloyed titanium-coluimbium alloy of Examples 3 and 4 with the colurnbium alloy not prealloyed with titanium of Examples 7 and 8, it Was found that the oxidation protective life at 2500 F. of Examples 3 and 4 was increased by a factor of 2 over the life of the alloys of Examples 7 and 8.

The hardness data indicated that titanium lowered the base metal hardness and chromium increased it. Even after 52 hours at 2500 F. the Cb Cr phase is quite stable and only a small increase in hardness has occurred beneath the visible Cb Cr phase in the diffusion zone indicated inward diffusion of very small amounts of chromium or oxygen.

Thus it will be appreciated that refractory metals and alloys may be prealloyed with titanium and coated in accordance with my invention with oxidation and resistant metals and alloys to obtain resultant alloys which have improved oxidation resistant properties heretofore con sidered unobtainable in the art and other improved properties as well.

The foregoing examples are intended as illustrations of my invention and although various minor modifications might be suggested by those versed in the art, it should be understood that I wish to embody within the scope of the patent warranted herein also such embodiments as reasonably and properly come within the scope of my contribution to the art.

I claim as my invention:

1. An oxidation resistant refractory article consisting essentially of a columbium base metal, a first diffusion zone consisting essentially of a titanium-columbium a1- loy bonded to said base metal, a second difiusion zone of chromium and said columbium base metal bonded to said first diffusion zone, and a layer of a cnomium and titanium alloy overlying said second diffusion zone.

2. The article of claim 1, which has a layer of silicon forming the outermostcoating of said article.

3. The article of claim 1 which has a third diffusion zone of chromium, titanium, and silicon overlying said second diffusion zone and an outer layer of silicon metal.

4. The article of claim 1 in which said columbi-um base metal has therein at least one alloying ingredient selected from the group consisting of 1 to 10% titanium, 1 to 15% molybdenum, 1 to 5% of zirconium, and 1 to 20% tungsten.

References Cited by the Examiner UNITED STATES PATENTS 2,865,088 12/1958 Yntema et a1. 29198 2,993,264 7/1961 Grenoble 29198 3,063,866 11/1962 Mayer et a1. 117-71 3,071,491 1/1963 Horn et a1. 29-498 3,077,421 2/1963 Budi-ninkas 117-71 3,078,554 2/ 1963 Carlson 29-194 3,081,530 3/1963 Wlodek 29-194 3,091,548 5/1963 Dillon 29198 3,108,013 10/1963 Cha-o et a1. 29-198 DAVID L. RECK, Primary Examiner.

WILLIAM D. MARTIN, HYLAND BIZOT, Examiners.

R. E. HOWARD, R. O. DEAN, Assistant Examiners. 

1. AN OXIDATION RESISTANT REFRACTORY ARTICLE CONSISTING ESSENTIALLY OF A COLUMBIUM BASE METAL, A FIRST DIFFUSION ZONE CONSISTING ESSENTIALLY OF A TITANIUM-COLUMBIUM ALLOY BONDED TO SAID BASE METAL, A SECOND DIFFUSION ZONE OF CHROMIUM AND SAID COLUMBIUM BASE METAL BONDED TO SAID FIRST DIFFUSION ZONE, AND A LAYER OF A CROMIUM AND TITINIUM ALLOY OVERLYING SAID SECOND DIFFUSION ZONE. 